|
|
Post by wickman on Nov 4, 2021 15:42:49 GMT
Hi! My listening position is 3-4 feet in front of the back wall. There is a large bookshelf propped against that wall, cluttered with books and other stuff. The bookshelf is about 1-2 feet deep. I was under the impression that it might work as a diffuser but reading here I now understand that's not the case. So... I'm thinking. Rather than getting rid of the entire bookshelf (which, btw, my wife made pretty clear is not an option. lol), would it be possible to purchase one of those "plastic" type 2d diffusers and just cut it up in two pieces horizontally and insert them snuggly against the back end of the bookshelf. That is, insert it inside the pieces into the shelves? I'm thinking a product like this, recommended by my HiFi guy.  |
|
|
|
Post by rock on Nov 5, 2021 17:32:08 GMT
I'm going to pass along some 2nd (or 3rd) hand info on diffusors so take this with a grain of salt  IIRC, the 3 to 4 foot range is on the side of being a little to close for diffusors, and for behind you, I believe 1D diffusors are either more applicable or more commonly used or both. The 2D seem to be better for ceiling...but see my opening line. Having said that, depending on other details about your listening room, you might be better off with absorption behind you. If your bookshelf is deep enough, you might try to fit some 4" thick mineral wool or rigid fiberglass in the back of your shelves and still have room for books and stuff in the front. BTW, I have used black economy landscape fabric as a covering for my absorbers. This is very thin. It keeps the loose fibers in but lets the air pass right through. www.menards.com/main/building-materials/landscaping-materials/landscaping-fabric/yardworks-reg-3-x-50-economy-landscape-fabric/00001/p-1444427384331-c-13236.htm?tid=-8515224090698823307&ipos=7 |
|
|
|
Post by Hexspa on Nov 6, 2021 6:48:22 GMT
I recall Ethan saying that distance to the diffuser didn't matter much in his subjective experience.
What I understand about 'duct tape' diffusion is that you're as likely to focus the sound as you are to diffuse it. Diffusion is a subset of scattering designed to specifically decorrelate reflected sound in time and space. Unless you can measure the results of your work, your home made diffuser might be more like a 'focuser' or magnifying glass to the sun.
In support of absorption, if your back wall is that close, then you might have other reflective surfaces nearby. This would mean you have more work to do than on just this wall. That said, you can take a good first step toward acoustically removing that wall by absorbing it. In other words, the more you absorb it, the more it disappears.
Something to consider is that diffusion doesn't eliminate acoustic interference - it minimizes the magnitude. To me, having a well-absorbed space is your foundation. On top of that, you can add subjectively-pleasing reflectiveness. If you have a mix of specular (flat) and diffuse reflections, it can be like putting lipstick on a pig. But, as I've mentioned, I don't have any experience with diffusers so maybe I'm wrong.
|
|
|
|
Post by wickman on Nov 7, 2021 20:34:25 GMT
Yeah, I saw the video where Ethan demonstrated that diffusors works well even when you're close up to it. In my case (4 feet), I'm betting it's far away enough. So that's basically where I got the idea from rock, you're probably right that a 2D diffuser would make more sense as a DIY to cut up and place inside the bookshelf because it's uniform. Good point. @hexpa: Yes you're right. I am in fact already adding absorbtion panels at the first reflections (right, left, ceiling + floor) so I don't wanna add more absorbtion unless I clearly have to. I may be wrong but I'm fairly certain that diffusion helps make the wall disappear and the room bigger? |
|
|
|
Post by Hexspa on Nov 8, 2021 9:01:37 GMT
How would diffusion make the wall disappear when it's sending back reflections? It makes the wall more complex. Absorption makes the sound not be able to bounce back. Hence, it's more like having no wall but rather an open window.
What I heard is that these diffuse reflections create interference and that's what we perceive as a wider sound stage. It's the same as specular reflections except with less magnitude because the sound is spread out. If the reflection was perfect, you wouldn't hear anything out of your speakers - just like flipping polarity in a daw. Of course, sound travels slower so, by the time it gets back to you, it's combining with a different signal. If those two later signals, direct and reflected, contain the same frequencies and the same amplitude but opposite polarity at the same time, they'll totally cancel. Since that's unlikely, you'll just get a reduction. If they're in phase, you'll get a boost - though the boost will always be less than the null.
It's like throwing a stick of butter at someone vs spraying them with Pam. Both will make them greasy but one looks better on camera. Flat reflections are the stick of butter and a spray nozzle is like diffusion for the liquid inside.
Why do you want to avoid too much absorption and how much do you think that is?
|
|
|
|
Post by wickman on Nov 10, 2021 10:09:15 GMT
How would diffusion make the wall disappear when it's sending back reflections? It makes the wall more complex. Absorption makes the sound not be able to bounce back. Hence, it's more like having no wall but rather an open window. Thanks! You're probably correct. It's just that I read that it makes it room sound bigger and, I mean, it's hard to know what's what really. Here is a reference if you're interested. youtu.be/seZyhcf1U3g?t=125Why do you want to avoid too much absorption and how much do you think that is? Again, you're probably correct. It's just that my room is very small and my biggest problem is with low bass at 43 Hz and some other issues which I need to treat. I realize that's where I should start. It's just that dealing with mid-highs (early reflections etc) is pretty easy in comparison to manage the crazy bass... and the panels are comparatively cheap vs bass traps. I'm thinking about using Roon EQ to drop the crazy spike at 43 Hz and then (somehow) add bass treatment. But I have absolutely no clue where to start. "Just get bass traps" Yeah, right getting 8 of those buggers are expensive and I wouldn't even know if they'll fit and what effect they would have. Here's the freq response for my room  |
|
|
|
Post by Hexspa on Nov 11, 2021 14:16:32 GMT
He says right in the video that too much diffusion in a small room is a mistake since adequate broadband is more appropriate.
We all have acoustically small rooms. A room is only considered not small when the volume is 10,000ft3 or more. The approach is always the same: use as much absorption as you can stomach then add more 10 years later.
Subtractive EQ is a good approach but best applied after speaker/listening position placement and absorption is installed.
As for where to start, Ethan has the gold standard articles which I followed (see the forum stickies). You're guaranteed improvement if you do the work. I also have my Acoustic Analysis For Drunks tutorial which you can find on this site [20230917 out of print]. The short of it is: empty the room, measure it, refine placement of speakers and listening position to flatten SPL, add absorption to minimize decay and comb filtering, add diffusion, apply EQ. Measure as you go to ensure you're on the right track. Post questions and review your materials regularly.
It's all good - don't stress. Small room acoustics is a manageable situation. It's going to cost money, time & effort but it's worth it.
|
|
|
|
Post by wickman on Nov 12, 2021 13:22:50 GMT
Subtractive EQ is a good approach but best applied after speaker/listening position placement and absorption is installed. The short of it is: empty the room, measure it, refine placement of speakers and listening position to flatten SPL, add absorption to minimize decay and comb filtering, add diffusion, apply EQ. Measure as you go to ensure you're on the right track. Post questions and review your materials regularly. Yes thanks all you say makes sense. I'll look into the docs you reference, although the crazy peak is not really fixed that easy I think. Just fyi, I'm very much stuck with the speaker/listening position as it is. The only place where the 43Hz peak is flat is in the middle of the room, which I cannot sit at since I would have the speakers in my lap more or less. |
|
|
|
Post by Hexspa on Nov 12, 2021 13:42:20 GMT
The peak is easily fixed - just high pass everything to 50Hz. Of course, that's an extreme approach. That said, the EBU spec for listening rooms only specifies down to 63Hz. In practice, though, the more flat bass extension you can get, the better.
The SPL response isn't the hard part, in any case. The hard part is the ringing. I'm not sure why you have what looks like a 20dB peak there. Either way, every small room needs absorption. If you can't move speakers around, your next step is to add panels.
Once you do that, take another measurement and see how much you improved things. Maybe you can't tame that peak - no problem. Do what you can and you can always use headphones to monitor your bass. It's not the end of the world; professionals do it every day.
|
|
|
|
Post by wickman on Nov 12, 2021 15:58:43 GMT
The peak is easily fixed - just high pass everything to 50Hz. Of course, that's an extreme approach. That said, the EBU spec for listening rooms only specifies down to 63Hz. In practice, though, the more flat bass extension you can get, the better. The SPL response isn't the hard part, in any case. The hard part is the ringing. I'm not sure why you have what looks like a 20dB peak there. Either way, every small room needs absorption. If you can't move speakers around, your next step is to add panels. Once you do that, take another measurement and see how much you improved things. Maybe you can't tame that peak - no problem. Do what you can and you can always use headphones to monitor your bass. It's not the end of the world; professionals do it every day. I realize that and thanks for you encouragement! But that stupid peak really ruins it for me as you can imagine. Lots of music are simply impossible to listen to, so that's my main issue. I'll fix it somehow eventually though... Anyway I think the peak is due to standing waves, room mode. The max Db of the frequency is at the each of the end of the room and disappears in the middle. It's quite interesting running a sine wave at 43Hz and then walk the rum. It's insanely high at the front 1-2 meters, then normalizes completely in the middle of the room and then picks up again at the 1-2 meters at the back. The room is calculated here and it correlates quite well. At least the first peak I was inspired by your youtube video and created a decay graph (140 - 160 ms is the blue graph) so you can see for yourself.   Btw, i don't get the reference to the EBU spec and 63Hz. Does it mean I should not try to fix anything below 63Hz? |
|
|
|
Post by Hexspa on Nov 13, 2021 5:07:54 GMT
Right. This is what I was talking about when I said 'ringing'. The dimensions of your room happen to support a 43Hz resonance. When you experience high sound pressure near the two walls, that's the effect of that axial mode. The bass is still there in the center of the room but it's in a velocity phase so you can't hear it. You hear sound pressure, not sound velocity. Incidentally, this is why sub bass is hard to absorb - the velocity phase is far from the boundaries and quite long. Frictional absorbers work on the velocity phase.
I don't think the EBU recommendation is to not worry at all below 63Hz. Maybe the reason they say that is twofold: 1. sub bass isn't going to mask the (highly translatable) midrange and 2. it's just not that easy to treat. You can put it a lot of absorption and still have some weird lumpy ringing in the sub range.
While that ringing is the attention-seeker of the show, it's all the nulling below it that is more of an issue and you can also fix. 65Hz and 80-90Hz are easier to treat and will make a bigger difference overall. People seem to get mesmerized by the big bad sub peak and totally ignore the rest. I was guilty of this too.
Just do what you can manage. Even ten pillows is better than nothing.
|
|
|
|
Post by wickman on Nov 13, 2021 15:38:42 GMT
While that ringing is the attention-seeker of the show, it's all the nulling below it that is more of an issue and you can also fix. 65Hz and 80-90Hz are easier to treat and will make a bigger difference overall. People seem to get mesmerized by the big bad sub peak and totally ignore the rest. I was guilty of this too. Just do what you can manage. Even ten pillows is better than nothing. Thanks again. As for fixing the dips at 65, 80-90 Hz (as you point out), what would you recommend, apart from ugly quick fix with EQ? From what I understand, you need to use tunable traps for specific narrow frequency intervals if you want to deal frequencies below 80'ish Hz like this one. But the dips though... quite counterintuitive that you can absorb away dips :-) It's not clear from Ethans texts where the Hz intervalls are for broadband DIY absorbers like the one in the link (the rigid fiberglass straddling corner-type). It says "quite effective to frequencies below 125 Hz", but hmm.... I guess not below 100Hz? |
|
|
|
Post by Hexspa on Nov 13, 2021 16:07:00 GMT
Dips are nulls aka nodes. They're caused by boundary interference. Best practice is to not boost them with EQ because A.) it'll put more strain on your loudspeakers and B.) you're working against nature; you can keep boosting but it'll keep nulling. Also, C.) SPL is location-dependent. Due to the complex interaction of waves in a room, your boost at 85Hz may raise a null slightly but it'll likely cause a peak just a short distance away from where you first measured. This is why you're supposed to measure in a reasonable radius (1-3') when assessing SPL.
Tuned resonators are definitely an option - after broadband. Broadband absorbers are, in fact, relatively effective below 125Hz. I say 'relatively' because it depends on the thickness and type of absorber. With fluffy, you can install 4'-thick and be totally absorbed down to about 31Hz. 8"-thick rigid should get you pretty well absorbed down to 63Hz. The reason that absorption coefficients below 125Hz aren't regularly featured is because it's not industry-standard practice to measure that bandwidth.
It might be counter-intuitive to think that absorbing will remove nulls but think again. If you put an audio file on two tracks with the levels identical and invert the polarity of one of them, what is the result? The result is zero output. Now mute the inverted track. Since there's signal again, you've added by subtracting. These nulls are exactly the same with one difference: though reflections are copies, they're changed in time and level. Hence, you don't get a totally zero null in a room.
|
|
